Think sensei Mike smith said somthing along those lines when i first asked him, why not use the shinai to reduce the risk of getting clonked.

reason being is to do with weight and the ability to make the cut. with a shinai, you only need the wrist- does not fit the whole body art thing.... Cut with the whole body, not glance using the wrist.

Being wacked by Sensei convinced me. I mean you see body movement with a bokken, there's warning but the shinai doesn't move, you just get this pain where it's hit.